From U.S. Pat. No. 3,621,382 an apparatus for measurement of magnetic fields is known in which a thin ferro-magnetic layer is used, which is easily magnetizable in one axis, the so-called easy axis, but more difficult to magnetize in an axis at right angles thereto, the so-called hard axis. The field, whose field strength is to be measured, acts upon the ferro-magnetic layer, and the apparatus includes a device for generating a symmetrical periodic alternating field, which field extends in a direction parallel to the easy axis of the ferro-magnetic layer, and wherein the alternating field is generated by a pumping coil. A detection coil is wound around the easy axis. A steady biasing field is applied along the hard axis, at least if the thin film is a flat strip. The value of the field component along the easy axis is deduced from the time difference between successive reversals of the magnetic vector in the strip from the amplitude of the harmonic 2f of the signal frequency f in the detection coil.
There is also known a current-measuring system of the type in which the magnetic flux from a current to be measured is superimposed upon the flux from a cyclically varying biasing current, and the flux reversals in the resulting flux are detected by using a thin film a few microns thick, of magnetic material, desaturation of which, as the flux reversals occur, is detected, either by the change of flux in the main magnetic circuit, or by the change in the resistance of the film, separate contacts being attached to the film for this purpose. The so-called easy axis of the magnetic film, alternately called preferential direction of magnetization, may be disposed parallel, at right angles to, or at an angle of, for example, 45.degree. , to the direction of the external magnetic field acting on the magnetic film. The magnetic coil for generating the premagnetizing field uses a flat coil made of a tape or wire, or a copper spiral in the form of an etched printed circuit.
From the IEEE transaction on Magnetics, Volume MAG-12, No. 6, November, 1976, pages 813-815, there is also known a linearly operating current detector, which includes four magnetic film pairs of magneto-resistive material in the form of a bridge circuit. The current to be measured generates a magnetic external field, directed in the direction of the hard magnetic axis of the magnetic films, and which in turn causes a steady rotation of magnetization of the magnetic films beyond their easy axis. This in turn results in a linear dependence of the resistances of the magnetic film within its effective controllable range, and consequently in a voltage, which can be obtained from the bridge circuit derived from the current to be measured.